Heat treatment of projectiles



Dec. 1, 1942; w. H. SODERHOLM 2,303,408

HEAT TREATMENT OF PROJECTILES Filed July 16, 1941 E m E m E E EQwucwvto'o Wnlcer 1-1. 5 m derhnlm m aw Srw

. expensive undertaking.

Patented Dec. 1, 1942 OFFICE HEAT TREATMENT OF PROJECTILES Walter H.Soderholm, United States Army,

Syracuse,

Application July 16, 1941, Serial No. 402,680

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 4 Claims.

The invention described herein may be manu factured and used by or forthe Government for governmental purposes, without the payment to me ofany royalty thereon.

My invention relates to the heat treatment of metals and to theproduction of a metallic body which is gradated in hardness or which iscommonly referred to as zone hardened.

An object of an embodiment of my invention is to provide an armorpiercing projectile having the proper gradation of hardness from thepoint and outer skin surface to a tough core.

Another object of my invention is to provide a quick inexpensive methodfor producing a gradation of hardness in a metal especially in an armorpiercing projectile.

Another object of my invention is to provide a method and apparatus forquickly heating and quenching a metallic body so that proper gradationof hardness exists throughout the body.

The nature and further objects of my invention will appear from thefollowing description and accompanying drawing.

It is known that various methods have been used to produce an armorpiercing projectile gradated in hardness. Those prior methods, asexemplified in U. S. Patent 1,381,633, I-Iadfield et al., issued June14, 1921, require a relatively long time and involve a great deal ofapparatus with the result that the manufacture of armor piercing shellsgradated in hardness has been an Also, most all of the prior methodsinvolved more than one heat treatment operation with the result that thegrain size of the shell material was unduly large; it is now known thatby controlling grain growth in a metal iin hardness can be controlledand the smaller the amount of grain growth the harder the material is.The prior art methods involved prolonged and numerous heating operationsduring which there was considerable grain growth and a related decreasefrom the optimum material hardness. Grain growth in carbon steels ispronounced at the hardening temperature and accordingly alloys whichserve as inhibitorsto grain growth have found extensive application inthe manufacture of armor piercing projectiles. Maintaining a metal atits hardening temperature for an appreciable time resuits inconsiderable grain growth and a relatively small degree of hardnessafter the metal is subsequently quenched. If grain growth in carbonsteels could be substantially prevented during the heat treatmentoperation a hard steel would be obtained and expensive alloys would nolonger be necessary in the manufacture of armor piercing projectiles.

In order to substantially prevent grain growth it is necessary to heatthe material to the hardening temperature in the shortest possible timeand then quench the material very quickly; this is accomplished by myapparatus. In order to produce a projectile gradated in hardness it ispractically necessary that there be difierential or zone heating ifsubstantial grain growth is to be prevented; in the prior art theinterior of the shell was heated, due to conduction, by heat supplied tothe external surface of the shell and this process of course requiredtime for the interior portions of the shell to be heated during whichthere was substantial grain growth in the outer shell material. In thisembodiment of my invention differential heating is accomplished by eddycurrents and to a smaller extent by the hysteresis losses in the iron.

In many of the prior art arrangements gradated or zone hardening wasaccomplished in two or more steps, i. e., initially the shell washardened uniformly and then by one or more steps the shell was againheated and portions thereof softened so that the tip and outer surfacesoi the shell would have a greater hardness than the inner core; thismethod requires many heat treatment operations and of course theopportunity for grain growth in the material is enhanced. In theembodiment of my invention there is but one heating operation and onequenching operation so that there is not much opportunity for graingrowth.

Referring to the drawing, in this embodiment of my invention the armorpiercing projectile It made of pure carbon steel or carbon steel mixedwith another metal is centrally disposed in a high frequency coil ormodified solenoid H which has an arcuately flared section I! disposedadjacent to the tip or ogival head of the projectile and a cylindricalsection 30 disposed adjacent to the cylindrical portion of the shell.High frequency currents are circulated through the coil or modifiedsolenoid II and magnetism due to that current causes eddy current lossesand hysteresis losses to be manifested as heat in the projectile. Theeddy current distribution in the projectile might be approximated by theformulae given by Steinmetz in Chapter 7 of his book TransientPhenomena. The energy which manifests itself as heat penetrates theouter surface of the projectile at the cylindrical portion approximatelyinversely proportional to the square root of the frequency. Bycontrolling the strength and frequency of the current in the coil theheat supplied and the depth to which the heat penetrates the projectileHI may be controlled and thus the projectile is differentially heated.The current is of such a value that the projectile is heated as rapidlyas possible to the quenching temperature.

It is a known fact that when a body having a relatively small projectionis heated there is a strong tendency for that projection to assume ahigher temperature than the surrounding material. This phenomenon isobserved especially in the case of cutting tools and for that reason acutting tool is not made with a cutting edge that is too pointed.Accordingly, when the projectile I is heated there is a tendency for itstip or ogival head to assume a higher temperature than the surroundingmaterial; this is desirable in the case of a projectile for theprojectile tip or ogival head ultimately should be harder than the bodyof the projectile and the projectile material should be progressivelyless hard as the distance from the pointed end of the ogival headincreases. In order that the tip or ogival head might not be overheatedby the combined effect of the heat due to the eddy current andhysteresis losses in the tip or ogival head and the heat which issupplied from the surrounding material in accordance with the abovephenomenon provisions are made so that the eddy current and hysteresislosses in the elemental parts of the tip or ogival head are not as largeas that in corresponding elemental parts of the body portion of theprojectile as measured from the projectile surface; this is accomplishedby spacing the adjacent coil windings a greater distance from the shelltip than the distance between the body of the shell and its adjacentcoil windings, i. e., the coil has an arcuately flared section i2 asindicated on the drawing and the tip of the projectile extends outsidethe arcuately end of the flared section a small amount whereby a greateramount of heat is generated in elemental parts of the body portion thanin corresponding elemental parts of the ogival head as measured from theprojectile surface.

The coil or modified solenoid I I is formed from a hollow water cooledconductor similar to the one described in the Northrup Patent No.1,328,336, issued on January 20, 1920 but it is understood that thecross section of the conductor may be solid, of "Litzdraht or may take avariety of forms and shapes.

The coil or modified solenoid l l is supplied from the high frequencysource I3 through the double pole double throw switch I. The contactsl5, IS on switch H are not energized.

The quenching fluid is supplied from a source II through gate valve l8and conduit i9. Gate valve I8 is normally closed and is opened whenplunger is moved due to the presence of a magnetic field in the coil 2|.Such valves are well known in the art. The coil 2| may be connected to adirect current source of supply 22 through the double pole double throwswitch 28. The contacts 24, 25 of switch 23 are not energized.

The control handles of switches I4 and 23 are rigidly connected togetherby the member 2 so that coils H and 2| cannot be energizedsimultaneously, i. e., the projectile cannot be heated and quenched atthe same time. The particular arrangement of the member 2 on theswitches serves another important function in that by a' ing fluid maybe caused to flow almost immediately after the projectile is heated tothe hardening temperature; thus, the time during which grain growth inthe projectile metal takes place is very short.

In the production of armor piercing projectiles, proper gradation ofhardness from the point and outer skin surface to a tough core isessential; such proper gradation of hardness consists not merely inproviding the projectile with a thin layer of hardened material but inproviding the projectile with portions having different degrees ofhardness along the projectile surface as well as in depth. The abovementioned HTadfleld et al. Patent 1,381,633 on page 2, lines 18 to 30relates to the degrees of hardness essential to the production of armorpiercing projectiles having the proper gradation of hardness from thepoint and outer skin surface to a tough core.

My invention has been disclosed as applied to the heat treatment of anarmor piercing projectile but my invention is not limited to thespecific embodiment which is disclosed but might be applied in the heattreatment of objects other than armor piercing projectiles; it isunderstood that changes in the precise embodiment of the inventionherein disclosed may be made within the scope of what is claimed withoutdeparting from the spirit of the invention.

I claim:

1. An induction heating apparatus for an armor piercing projectilehaving a body portion and an ogival head, an electrical air coremodified solenoid having a cylindrical section adapted to have insertedtherein said body portion and to encircle said body portion, and saidmodified solenoid having a flared section which is adapted to at leastpartially encircle said ogival head, said flared section being arcuatelyflared to such a degree whereby less unit heat is produced in the ogivalhead than in the body portion as measured from the projectile surface.

2. In the process of producing a zone hardened steel projectile gradatedin hardness throughout its interior and in accordance with apredetermined pattern, said projectile having a body portion and anogival head, the steps which consist in quickly difl'erentially heatingelemental parts of the projectile with correspondingly more heatsupplied to the elemental parts of the body portion than thecorresponding elemental parts of the ogival head as measured from theprojectile surface, and then quickly quenching the projectile, whereby aprojectile gradated in optimum degrees of hardness throughout theinterior portions of the ogival head and body portion is produced withbut one heating and quenching operation and with elemental parts of theogival head correspondingly harder than elemental parts of the bodyportion as measured from the projectile surface.

3. In the process of producing a zone hardened steel shell having anogival head and a body portion, the steps which consist in quicklysupplying more heat per elemental part to the body portion than tocorresponding parts of the ogival head as measured from the shellsurface, said heat being supplied by differentially heating elementalparts of the body portion and differentially heating elemental parts ofthe ogival head, and then quickly quenching the body portion and ogivalhead whereby a shell gradated in optimum degrees of hardness throughoutits interior is produced with but one heating and quenching operationand with elemental parts of the ogival head corresponddegree wherebyless unit heat is generated in the elemental parts of the oglval headthan in corresponding elemental parts of the body portion as measuredfrom the projectile surface, and then quickly quenching the projectilewhereby a pro Jectile is produced gradated in a plurality of stepsnon-uniformly throughout the projectile body and with elemental parts ofthe oglval head harder than corresponding elemental parts of the body 10portion as measured from the projectile surface.

WALTER H. SODERHOLM.

